Retractable safety belt

ABSTRACT

A seat belt retractor having an automatic clamp for preventing extension of seat belt employing a moving clamp part which also acts as a buffer to prevent the retraction reel from taking up increments of slack which occur while the belt is fastened due to abnormal movements of the user.

' United States Patent [191 Board et a1.

[11] 3,817,474 i451 June 18, 1974 1 1 RETRACTABLE SAFETY BELT [76]Inventors: Richard G. Board, 3000 Connecticut Ave., Washington,

DC. 20008; NelsonH. Shapiro, 640

Washington Bldg., Washington, DC 20005 221 Filed: Apr. 1, 1965 211 App].No.: 444,658

[52] US. Cl. 242/ 107.2, 242/107 SB, 297/388 [51] Int. Cl A62b 35/00[58] Field of Search... 297/388; 242/107 SB, 107.2,

[56] References Cited UNITED STATES PATENTS 3,205,004 9/ l 965 Spouge297/388 3,227,489 1/ 1966 Stubblefield 297/388 3,246,929 4/1966 Taggart297/388 FOREIGN PATENTS OR APPLICATIONS 7/191 1 Switzerland 242/107.2

Primary Examiner-Wemer 1-1. Schroeder Attorney, Agent, or Firm-Shapiroand Shapiro 1 1 ABSTRACT A seat belt retractor having an automatic clampfor preventing extension of seat belt employing a moving clamp partwhich also acts as a buffer to prevent the retraction reel from takingup increments of slack which occur while the belt is fastened due toHQXQEBFQLQflhEEQ:

2 Claims, 13 Drawing Figures PM'ENYEDJUN I 8 I974 SHEU 1 BF 3 INVENTORSRICHARD G. BOARD NELSON H. SHAPIRO BY Ska 00b a/lci ka 0170 ATTORNEYSPMENTEDJUN 1a 1914 snmzora- FIG. 5

FIG. 5

v INVENTORS RICHARD G. BOARD NELSON H. SHAPIRO BY \S IOPI IO and SAQIOATTORNEYS INVENTORS RICHARD 6- BOARD NELSON H. SHAPIRO BY SA 0H0 'cmciSka aim ATTORNEYS r 1 'RE'I'RACT ABLE SAFETY BELT This invention relatesto retractable safety belts and more particularly to so-calledretractable seat belts for use in automotive vehicles.

Copending applications, Ser. No. 417,924, filed Dec. 14, I964, Ser. No.409,266, filed Nov. 5, 1964, and Ser. No. 377,344, filed June 23, 1964,all entitled RE- TRACTABLE SAFETY BELTS disclose and claim retractableseat belt apparatus in which a strap having a fastener member at a freeend thereof may be grasped by the user and extended from a retractiondevice to place the strap in user-restraining position, whereuponfurther extension of the strapis prevented automatically in response,for example, to the cessation of extensile movement or to slightretractile movement of the strap. At the user-restraining position thestrap may have any selected length within a wide range of lengths, so asto accommodate varying conditions of user size and clothing. Thefastener member is engaged with a mating fastener member in order tocomplete the belt. When the fastener members are disengaged, the strapretracts automatically. The present application discloses and claimsimprovements in and modifications of such apparatus.

It is accordingly a principal object of the invention to further.extension, and retracted automatically when released.

Still another object of the invention is to provide im-. provedapparatus for retractable seat belts and the like which is responsive tochanges in tension of the strap.

The applicants have previously proposed retractable seat belt apparatusand'the like having a member past which a strap is trained, the memberbeing moved automatically to a first position when the strap isretracted, remaining in that position during extension of the'strap',and moving automatically to a second position when extensile movement ofthe strap ceases. At the second position the member serves to preventfurther extensile movement of the strap. It is a principal object of thepresent invention to provide improved retractable seat belt apparatusand the like employing this principle.

Still another object of the invention is to provide retractable seatbelt apparatus of the foregoing type whichutilizes variations in theforces in the strap extending from the aforesaid member to provide thedesired operation.

Yet another object of the invention is to provide an improvedretractable'seat belt and the like employing a feeler mechanism forsensing the amount of strap wound upon a retraction reel and forcontrolling the operation of the aforesaid member.

still another object of the invention is to provide retractable seatbelt apparatus and the like having im proved means for preventing theextensile movement of a strap after the strap has been placed inuserrestraining position.

Applicants have previously proposed retractable seat belt apparatus inwhich extensile movement of a strap is prevented by the closing of aclamp after the strap has been extended to the desired user-restrainingposition and in which the clamp opens automatically during retraction ofthe strap, remains open until the strap has been extended to the desiredlength, and then closes automatically.

It is a principal object of the present invention to provide improvedapparatus of the foregoing type.

Another principal object of the invention is to provide apparatus ofthis type which requires only one simple moving part.

A possible objection to the use of automatic clamps for preventingextension of a seat belt is that the normal movement of the user,particularly as the vehicle rides over bumps on the road, may tendtocreate increments of slack which are immediately taken up by theretractor, progressively tightening the belt to an undesirable extent.It is accordingly a principal object of the invention to provideapparatus which alleviates this problem.

More specifically, it is an object of the invention to provide apparatusemploying a moving clamp part which also acts as a buffer to prevent theretraction reel from taking up increments of slack which occur whil thebelt is fastened.

Briefly stated, but without limitation, the present invention utilizes aclamp having a moving clamp part and a fxed clamp part. A strap istrained from a retraction reel between the clamp parts and about themoving clamp part in such a way that forces exerted by the user upon thebelt after the belt has been fastened close the clamp to preventextension of the belt, and forces exerted upon the moving clamppan bythe reel open the clamp during retraction. In addition to pivotalmovement tov provide the clamping function, the moving clamp part issupported for translational movement to provide a disabling function,that is, to prevent the closing of the clamp during extension of thestrap to the desired user-restraining position Furthermore, by virtue ofthe manner in which the strap is trained about the moving clamp part, abuffering functionis provided which prevents the reel from taking upincrements of slack when the belt is fastened.

The foregoing and other objects, advantages, and feature of theinvention, and the manner in which the same are accomplished, willbecome more readily apparent upon consideration of the followingdetailed description of the invention taken in conjunction with theaccompanying drawings, which illustrate preferred and exemplaryembodiments, and wherein:

FIG. 1 is a fragmentary prespective view illustrating the externalappearance of housing, strap, and fastener components of a typicalembodiment of retractable seat belt apparatus in accordance with theinvention;

FIGS. 2 6 are longitudinal sectional views illustrating differentpositions assumed by a moving clamp part of the invention in the courseof the operation of the invention;

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6;

FIG. 8 is a perspective view of a moving clamp part employed in theinvention;

FIGS. 9 12 are longitudinal sectional views illustrating differentpositions of a moving clamp part in accordance with a modification ofthe invention; and

FIG. 13 is a perspective view of a modified moving clamp part.

Referring to the drawings, and initially to FIG. 1 thereof, referencenumeral 10 designates a conventional seat belt fastener couplecomprising a buckle l3 and a tongue 14. The fastener parts may have theusual construction, the buckle comprising a housing having an accessopening at the front for reception of the tongue and including the usuallocking abutments which enter the openings 16 of the tongue so as toretain the tongue in the buckle until the buckle release lever 18 isactuated. A strap 20 affixed to the buckle may have its distal endanchored to the floor of the vehicle at one side of the seat, forexample. This strap may be kept quite short, may be rigidified, and neednot be adjustable. Strap 22, such as nylon webbing, is affixed to thetongue and is arranged to be retracted and extended with respect to aretraction device 24, which may be a spring-wound retraction reelsupported for rotation in a housing or casing 26 anchored to the floorof the vehicle at the opposite side of the seat, for example. Thehousing may be appropriately contoured to minimize space requirementsand for aesthetic reasons and it may have an opening 28 through whichthe strap 22 moves and at which the tongue 14 is located when the strapis fully retracted. This opening may, in effect, be extended toward theuser by the use of a conventional boot or tube which guides the strap tothe rest position of the tongue.

As is well known in the art, the reel 24 (FIG. 2) may comprise a tubularhub 30 having bearings (not shown) at each end for supporting the hubfor rotation upon a pin 32 extending through the hub and fixed betweenside plates 34 of a frame, which may be part of the housing 26. Ahelical torsion spring 36 is received loosely within the hub between thehub and the pin 32 and has one end fixed to the pin and the other endfixed to the hub, so that the spring is wound when the reel is turned ina direction to extend the strap. Other types of reels, such as thoseusing spiral clock springs at the ends, may also be used. The sideplates 34 may be joined by spacers 38. It will thus be apparent that thetongue 14 may be grasped to extend the strap 22 from the housing 26,turning the reel 24 and winding the spring 36, and that when the tongueis released the reel will be turned by the spring, winding up the strapso as to retract it into the housing.

In accordance with the invention a clamp is employed for preventingextension of the strap when the strap has been extended to the desireduser-restraining position. The clamp comprises a moving clamp part 40and a fixed clamp part 42. The fixed clamp part may simply be a bar orrod extending between and mounted at its ends upon the side plates 34.As shown in FIG. 8, the moving clamp part may be a suitably shaped plateof generally rectangular outline provided with a pair of laterallyextending pivot pins 44, which may be integral with the plate. The pivotpins extend into slots 46 of the side plates 34 with sufiicient play topermit pivotal and translational movement of the pins. In the form shownin FIG. 2, for example, each slot is L-or chevronshaped having legsforming a right angle the bisector of which is substantially horizontal.One of the legs extends generally toward the center of the reel.

In the form shown the moving clamp plate has a generally concave-upwardcurvature, although the shape of the plate may be varied somewhat, aswill be described more fully hereinafter. Both the lower or leading edgeand the upper or trailing edge of the plate are shown smoothly curved,the strap being trained from one side of the reel past the upper edge ofthe plate, between the plate and the fixed clamp part 42, along theouter surface of the plate, around the lower edge of the plate, and thenupwardly at an appropriate angle past the other side of the reel. Thegenerally upwardly directed surface at the lower edge of the clamp platemay be flattened (at least at the lateral extremities of the plate) asshown at 48 in FIG. 8. These surfaces are adapted to engage stop pins 50which project laterally inward from the side plates (See FIG. 7).

In the position of the clamp plate shown in FIG. 5 the pivot pins 44 areat the top of the slots 46, and the pres sure exerted upon the lower endof the clamp plate by the tension in the strap leading to the tongue 14turns the clamp plate so that it clamps the strap against the fixedclamp part 42, as shown in FIG. 6. The clamping pressure is determinedby the relative length of the lever arms between the pivot pins and thelower edge of the clamp plate and between the pivot pins and the portionof the clamp plate adjacent to the fixed clamp part. This may be afavorable ratio of the order of 2 to l, 3 to l, or even more. Theclamping pressure is also determined by the angle of pull of the portionof the strap extending to the tongue with respect to the lever armextending between the lower edge of the clamping plate and the pivotpins. This angle may be of the order of for example. The harder the pullupon the strap, the tighter the clamping. The actual clamping surfacesmay be smooth, or one or both of them (preferably just the fixed part,which does not normally engage the strap during movement thereof) may beroughened or knurled. The upper edge of the clamp plate indents thestrap, as shown in FIG. 6, at least when the clamp is closed.

If the belt is unfastened, the pressure of the strap upon the upper edgeof the clamp plate due to the spring force of the reel will turn theclamp plate so as to open the clamp, as shown in FIG. 5, and the strapwill retract. If when the strap is substantially fully retracted thestrap tension is substantially removed, the clamp plate will be free toslide along the slots under the influence of gravity to the position ofFIG. 2. This may be facilitated by weighting the lower end of the plate,by having the reel spring fully unwound when the strap is fullyretracted or by having the webbing on the reel press against a portionof the housing when the strap is fully retracted, so as to stop themovement of the reel. FIG. 2 illustrates the fully retracted condition,it being noted that the clamp plate 40 has slipped to the bottom of theslots 46 and that the webbing is relatively slack.

If the strap is grasped and pulled from the housing, the pressure on theclamp plate 40 will cause it to turn to the position of FIG. 3, at whichthe flattened portions 48 engage the stop pins 50. The pressure of theflattened portions 48 against the stop pins and the pressure of thepivot pins 44 on the edge of the slots 46 will immobilize the clampplate in an open clamp position while the tension in the strap ismaintained during extension of the strap to the user-restrainingposition.

If now the strap is relaxed slightly, as occurs during the fastening ofthe belt, the clamp plate will turn slightly more open because of thepressure of the strap upon the upper edge of the plate. The pivot pins44 will roll slightly upon the upper edges of the slots and will moveslightly up the slots, engaging the curved lower edge of the clamp platewith the stop pins 50 and camming the clamp plate about the stop pins asshown in FIG. 4, so as to accelerate the upward movement of the clampplate. The upward pressure exerted by the strap on the clamp platecauses the pivot pins 44 instantaneously to slide along the slots to theposition of FIG. 5, any slack in the belt created by this movement ofthe clamp plate being taken up by the reel. Any further extensile pullexerted upon the strap causes the clamp to close immediately as shown inFIG. 6, the clamp plate being freeof obstruction by the stop pins.

If now the belt is unfastened, the pressure exerted upon the upper edgeof the clamp plate by the webbing causes the clamp plate to turn to theopen position of FIG. 5, permitting the strap to be retracted upon thereel. It is only when the tension in the strap is substantially removed,as shown in FIG. 2, that the clamp plate will again travel underv theinfluence of gravity down the slots. Otherwise, the strap tension issufficient to maintain the clamp plate at the top of the slots.

FIGS. 5 and 6 illustrate another useful characteristic of the invention,whereby a buffering action is provided so as to avoid taking 'upincrements of slack created during normal use with the belt fastened. Ifthe belt is tightened sufficiently to move the clamp to the closedposition of FIG. 6, the upper edge of the clamp plate will substantiallyindent the webbing laterally, the reel turning in a clockwise directionin FIG. 6 to provide the webbing for the indentation. If now the belt isrelaxed somewhat, as when the user moves more deeply into the seat, theclamp plate will turn toward the position of FIG. 5, and the reel willturn in a counterclockwise direction to absorb the slack created as theindentation is removed. However, there will be no actual movement of thestrap across the surface of the clamp plate, because the turningmovement of the clamp plate will cause the lower edge of the clamp'plateto indent the strap more deeply. it is only after the clamp plate hasturned to the open position of FIG. 5 that further relaxation of thebelt will cause slack to be taken up by the reel. Otherwise, when theuser moves back up relative to the seat, the clamp plate will merelyturn back to the position of FIG. 6 withdrawing some webbing from thereel to .create indentation at the upper edge.

The configuration of the slots 46 is such as to ensure that the clampplate moves behind (under) the stop pins 50 as it moves down the slotsand clears the stop pins as it moves up the slots. The upper leg of theslots is generally radial to the reel, and the lower leg is generallytangential. The upper edge of the lower leg assists the stop pins inmaintaining the position of the clamp plate during extension of thestrap. The effect of the slots is to permit the clamp plate to movebetween a first position, at which turning is obstructed by stop pins50, and a second position at which turning is unobstructed. The clampplate thus becomes its own latch." The pivot pins may be enlarged forstrength and the slots enlarged accordingly. However; it is notnecessary that the upper leg of the slots be long enough to accommodatethe whole pivot pin. The pin may protrude below the bend in the slotwhen the pin is at the top of the slot. Moreover, the pins which ridethe slots may be mere guide pins; the actual pivot pins may be largerand concentric with the guide pins, engaging suitable bearing surfacesfixed to the side plates when the guide pins are at the top of theslots.

FIGS. 9 l3 illustrate a modification in which the moving clamp part 52is moved to its rest position by the pressure of the webbing wound uponthe reel, rather than by gravity. It will also be observed that themoving clamp part, while a generally rectangular plate, has a differentconfiguration from the part 40 previously described in that the lowerportion of the clamp is upwardly convex and the upper portion of theclamp is essentially flat. Pivot pins 54 fixed to the sides of the clampplate are mounted for free movement in slots 56 in the side plates. Theslots have a different configuration from those previously described,being generally crescent shaped. The outer slot edge is smoothly curved,while the inner edge (toward the reel) has a steppedconfiguration toprovide a shelf on which the pins rest during extension of the strap.The orientation of the slots is somewhat different in this embodiment,the bisector of the angle formed by the slots being directed upwardly atabout thirty degrees in the general direction in which the strap ispulled. The upper surface of the clamp plate has a generally cylindricalfeeler or pusher member 58 afiixed thereto slightly below the pivot pins54 and adapted to engage the webbing wound upon the reel when the strapis retracted. Adjacent to its lower edge the clamp plate is formed atits lateral extremities with shallow notches 60, which are adapted toengage and receive stop pins 62 projecting slightly inwardly from theside plates 34. The clamp plate has an enlargement 64 at its upperextremity for engaging the webbing wound upon the reel when the reel isalmost full, as shown in FIG. 9. The fixed clamp part 66 in thisembodiment is shown as a cylindrical pin extending between the sideplates.

When the strap is fully retracted, the movable clamp part will have theposition shown in FIG. 9. When the strap is extended, the lower edge ofthe clamp plate will move slightly, due to the drag of the strap overthe clamp plate on its outer surface as well as on the surface of pushermember 58, so as to seat the stop pins 62 in the notches 60. In thismovement of the clamp plate the pivot pins 54 will move to the bottom ofthe slots 56 as can be seen from FIGS. 9 and 10. The clamp plate willremain in the position of FIG. 10, as long as the strap is pulled duringextension of the strap to the user-restraining position. If now thestrap is relaxed slightly, during the fastening of the belt, the movableclamp part will snap around the stop pins, as shown in FIG. 11 and asdescribed in connection with the previous embodiment, and will slideupwardly to the top of the slots at which any further pull upon thestrap will cause the clamp to close as shown in FIG. 12, preventing anyextension of the strap. If now the belt is unfastened, the clamp willmove to the open position of FIG. 11, as previously described. As thewebbing winds upon the reel, it will finally engage the pusher member 58and will press the clamp plate downwardly along the.

slots almost to the bottom thereof. At this time the enlargement orpusher element 64 of the upper edge of the clamp plate will engage thewebbing upon the reel, turning the clamp plate slightly clockwise asseen in FIG. 9 so that the notches approach the stop pins 62.

The same buffering action described previously is also available in theembodiment of FIGS. 9 l3, pres sure on the upper edge of the clamp plateby the strap acting as a return spring to turn the clamp plate to itsopen position. It is also to be noted that in this embodiment if only aslight amount of webbing is withdrawn from the reel before the clamp ispermitted to close, the pusher member 58 may engage the webbing upon thereel. In this event, the pusher member, riding upon the webbing, mayserve as the fulcrum for the clamp until the webbing upon the reel iscompressed sufiiciently to permit the pivot pins to reach the tops ofthe slots, but clamping of the strap nevertheless occurs.

The undersurface of the lower edge of the clamp plate at its lateralextremities may be contoured, as may the upper surface of the stop pinsto facilitate camming of the movable clamp part around the stop pins tothe rest position of FIG. 9 in the event the clamp is not fully openduring retraction of the strap. This is significant in this embodiment,because the lower edge of the clamp plate must be close enough to thestop pins to engage them promptly when the strap is initially pulled, inorder to prevent undesired pressure of the pusher member against thestrap on the reel. Furthermore, instead of having stop pins projectinginwardly from the side plates of the frame, the lower edge of the clampplate may have laterally projecting stop pins which move in suitablyshaped windows in the side plates, the windows having shelf portions toengage the upper surface of the stop pins at the initial position of theclamp plate.

If the reel has end discs, as shown at 68, the upper edge of the clampplate may rub gently against the end discs during retraction of thestrap until the last turn expands beyond the end discs to engage theupper edge of the clamp plate as in FIG. 9. If the upper edge of theclamp plate has an enlargement fitting between the end discs, the sameengagement can be obtained without expansion of the last turn of webbingbeyond the end discs. Moreover, in addition to or instead of end discson the reel, the strap may be guided by the side plate 34 or bossesthereon.

The amount of translational movement of the clamp plate (broadly,movement in addition to the pivotal clamping movement) need be onlysufficient to distinguish between the obstructed and unobstructedpositions of the clamp plate. The amount of turning or pivotal movementof the clamp plate is influenced by the amount of buffering actiondesired and may be quite small in the absence of buffering. The totalapparatus may be kept quite compact by suitably shaping the clamp plateand by suitably positioning the clamp plate in the housing. The clampplate may be made as thick as necessary to take collision loads.

The pusher member in FIGS. 9 13 has been located to ensure that theclamp plate is pushed in an open position, but other locations of thepusher member are possible, if correlated with other parameters, such asthe slot orientation relative to the reel.

The configuration of the clamp plate influences the friction dragapplied to the strap, and, of course, the reaction of the clamp plate tothe strap tension forces, which provide clamp opening and closingpivotal movement as well as latching and unlatching action. Friction canbe reduced by the use of appropriate contours or by the use of rollers,but the latter introduces complication.

Slot shape and orientation are determined by the requirements oftranslational movement of the clamp plate between obstructed (latched)or unobstructed (unlatched) positions. Slot components radial to thereel can be utilized to provide translational movement in response toexpansion of the wound webbing, while tangential components may beutilized to provide translational movement in response to drag exertedby the strap (which may also provide turning movement). The edges of theslots may provide cam surfaces for guiding the translational movement orto support tuming or rolling movement of the pivot pins. Slot shelvesare useful in maintaining the position of the pivot pins during strapextension, although the engagement of the clamp plate with stop pins orthelike may be sufficient in itself to prevent closing of the clampduring extension of the strap.

While preferred embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that changescan be made without departing from the principles and spirit of theinvention, the scope of which is defined in the appended claims. Forexample, other forces or combinations of forces may be used to move theclamp plate to its initial position and/or to hold it there. Drag of thestrap across the clamp plate, when the strap is slightly retracted afterfastening, may be used to release the latch. The clamp plate may beformed with a channel to guide the strap across its surface, or may havelateral extensions passing into receptacles in the side plates to engagerecessed stops. Accordingly, the foregoing embodiments are considered tobe illustrative, rather than restrictive of the invention, and thosemodifications which come within the meaning and range of equivalents ofthe claims are intended to be included therein.

The invention claimed is:

1. In a retractable belt having a retraction device with a strapconnected thereto and adapted to be retracted, extended to a desiredlength and then prevented from substantial further extension, the imrovement which comprises a strap guide supported or pivotal movementrelative to said device and having leading and trailing edges spaced onopposite sides of its pivotal axis, and means training said strap fromsaid retraction I device and reversely around said guide, withindentations at said edges, for exerting forces on said guide andcausing it to turn in one direction in response to the retracting forceof said retraction device exerted upon the trailing edge of said guideand to turn in the opposite direction in response to a force exertedupon the leading edge of said guide through the strap beyond saidleading edge, the indentation at one edge increasing as the indentationat the other edge decreases, and vice versa, whereby the effectivelength of the strap beyond said leading edge may decrease and thenincrease agan by an amount determined by the turning of said gui e.

2. In a retractable belt having a retraction device with a strapconnected thereto and adapted to be retracted, extended to a desiredlength and then prevented from substantial further extension, theimprovement which comprises a strap guide supported for pivotal movementrelative to said device and having leading and trailing edges spaced onopposite sides of its pivotal axis, and means training said strap fromsaid retraction device and reversely around said guide, in frictionalengagement therewith, for exerting forces on said guide and causing itto turn in one direction in response to the retracting force of saidretraction device exerted upon the trailing edge of said guide and toturn in the opposite direction in response to a force exerted upon theleading edge of said guide through the strap beyond said leading edge,the guide being pivotally movable through a predetermined distance withbelt movement in one direction, and vice versa, whereby the effectivelength of the strap beyond said leading edge may decrease and thenincrease a ain by an amount determined by the tuming of saic? guide.

1. In a retractable belt having a retraction device with a strapconnected thereto and adapted to be retracted, extended to a desiredlength and then prevented from substantial further extension, theimprovement which comprises a strap guide supported for pivotal movementrelative to said device and having leading and trailing edges spaced onopposite sides of its pivotal axis, and means training said strap fromsaid retraction device and reversely around said guide, withindentations at said edges, for exerting forces on said guide andcausing it to turn in one direction in response to the retracting forceof said retraction device exerted upon the trailing edge of said guideand to turn in the opposite direction in response to a force exertedupon the leading edge of said guide through the strap beyond saidleading edge, the indentation at one edge increasing as the indentationat the other edge decreases, and vice versa, whereby the effectivelength of the strap beyond said leading edge may decrease and thenincrease again by an amount determined by the tUrning of said guide. 2.In a retractable belt having a retraction device with a strap connectedthereto and adapted to be retracted, extended to a desired length andthen prevented from substantial further extension, the improvement whichcomprises a strap guide supported for pivotal movement relative to saiddevice and having leading and trailing edges spaced on opposite sides ofits pivotal axis, and means training said strap from said retractiondevice and reversely around said guide, in frictional engagementtherewith, for exerting forces on said guide and causing it to turn inone direction in response to the retracting force of said retractiondevice exerted upon the trailing edge of said guide and to turn in theopposite direction in response to a force exerted upon the leading edgeof said guide through the strap beyond said leading edge, the guidebeing pivotally movable through a predetermined distance with beltmovement in one direction, and vice versa, whereby the effective lengthof the strap beyond said leading edge may decrease and then increaseagain by an amount determined by the turning of said guide.